An image reading device such as a scanner, facsimile machine, multi-function office machine, and copy machine etc. needs a linear light source to illuminate the targets while liquid crystal modules needs to have an illuminated background. The method utilizes a light source combined with a light-guide bar so as to transfer the light source into a linear light source, in this way, the backlight can be provided for a liquid crystal panel.
Conventional linear light source devices for image reading are as follows: (1) utilize a cold cathode ray tube (CRT) as the linear light source 110 as shown in FIG. 1; (2) utilize a light-emitting diode (LED) array for the linear light source 120 as shown in FIG. 2; (3) utilize an LED array plus prismatic lens for the linear light source 130 as shown in FIG. 3; (4) utilize a prismatic light-guide bar for the linear light source 140 as shown in FIG. 4; (5) utilize a prismatic light-guide bar plus a housing for the linear light source 150 as shown in FIG. 5A and FIG. 5B; (6) utilize a light-guided bar for the linear light source 160, 170, 180, and 190 made up of the structure formed by the intersection of prismatic column planes and oblique planes as shown in FIG. 6, FIG. 7, FIG. 8A, FIG. 8B, FIG. 9A, and FIG. 9B.
The above-mentioned linear light source 110 shown in FIG. 1 is composed of a cold CRT 112 and a transfer circuit 114. The luminescence of the cold CRT 112 is the same as that of the fluorescent tube (hot CRT) except that it is suitable for small tube diameter since it has simple structure and compact electrode. But this technology needs to have a transfer circuit provided, making the overall size relatively large. Since the cold CRT 112 is a cylindrical type of luminaire, the rate of light energy utilization is low and it is unable to generate light with different wave-lengths. Additionally, it is fragile and has a short lifetime.
As shown in FIG. 2, a number of LEDs 124, such as forty pieces, are mounted on a substrate 122. The rate of light energy utilization is low since the space angle of the radiation can be greater than a half of a space. Moreover, since spaces exist between the LEDs, and since it is inconsistent in luminous intensity, the light uniformity is poor. Further, since a large number of LEDs 124 are required, the cost is high.
As shown in FIG. 3, many a quantity of LEDs 124, such as forty pieces, are mounted on a substrate 122. A prismatic lens 132 is also provided. In contrast with FIG. 2, although the addition of this prismatic lens 132 improves the rate of light energy utilization and light beam uniformity, the rate of light energy utilization is still low. Besides, it has the demerit of being high in cost.
Another conventional technology is shown in FIG. 4. The light is transmitted by the use of a prismatic light-guide bar 142 whose cross-section can be a circle, a rectangle, a triangle, an ellipse, or an irregular shape etc. The incident light 200 having an incident angle greater than the critical angle transmits into the light-guide bar 142 by total reflection without a loss in radiant flux, then goes through the light-guide bar 142 and exits out through the light-exiting plane to become an exiting light beam 202. As the light falls on the stripes of the surface, light having an incident angle smaller than the critical angle, refract from the stripes 144 of the surface to become out-refracting light 204. In the meantime, in contrast to the surface with stripes, the light also falls on the smooth surface. Those light beams having an incident angle smaller than the critical angle also refract from the smooth surface to become out-refracting light 206. Since the light-guide bar 142 is merely a simple prismatic column and the surface stripes 144 are in a simple belt-shape, the light uniformity is poor.
Another conventional technology is shown in FIG. 5A and FIG. 5B. A linear light source 150 is composed of a light-guide bar 152, a light source assembly 300, and a housing 158. The cross-section of the light-guide bar 152 show the prismatic column is a pentagon by cutting a corner of a rectangle or a polygon by cutting two or more corners of a rectangle. The plane formed by cutting an angle is a light-exiting plane 154. The side surfaces other than the two neighboring side surfaces between the light-guide bar 152 and the light-exiting plane 154 are coated with reflective layers 156 (see FIG. 5B). A housing 158 is provided, separating by a thin air layer, between the light-exiting plane 154 and at least a plane other than an end plane of the two end planes provided by the light source assembly 300. This kind of technology not only increases the size of the device a but also the cost since the housing 158 is required. The device is apt to generate a light beam having its incident angle smaller than the critical angle. Moreover, the light beam reflected from the plane of the light-guide bar 152 allows only a portion to be reflected from the inner wall surface of the housing 158 and is then refracted back again into the light-guide bar 152. As a result, the rate of light energy utilization is not high. Further, since the device depends merely on the reflective layers 156 to adjust the output radiant flux distribution, the uniformity is not sufficient.
Another technology is shown in FIG. 6 and FIG. 7. As shown in FIG. 6, a linear light source 160 is composed of a light-guide bar 162 and a light source assembly 300. The cross-section of the light-guide bar 162, constituted by the intersection of a prismatic column and an oblique plane, is a rectangle. Surface stripe 165 and reflective layers 166 are provided on an oblique surface 164 on the oblique plane, and the light-exiting plane 178 is opposite to the oblique surface 164. The linear light source 170 is composed of a light-guide bar 172 and two-end light source assembly 300. As shown in FIG. 7, the light-guide bar 172 is constituted by the intersection of a prismatic column and an oblique plane. The cross-section of the prismatic column is a rectangle and the oblique plane has two oblique surfaces 174. A light-exiting plane 178 is provided opposite to the oblique surfaces 174. The remaining setup is the same as those in FIG. 6. Since the cross-sections of the light-guide bars 162 and 172 are rectangular, light having an incident angle smaller than the critical angle is easy to generate. As a result, there is a loss of radiant flux making the rate of light energy utilization poor. Since the device depends merely on the linear variation of oblique planes 164, and 174 as well as the adjustment of the output radiant flux distribution, the light beam uniformity is poor.
In FIGS. 8A and 8B, a linear light source 180 is composed of a light-guide bar 182 and light source assemblies 300 positioned at both ends. The light-guided bar 182 is constituted by the intersection of a prismatic column and an oblique plane. The cross-section of the prismatic column is an irregular shape (see FIG. 8B). The oblique planes are made up of pair of two oblique surfaces 184, of the light-guide bar 182, inclined in opposite directions. The light-exiting planes 186 are other prismatic column planes of non-cylindrical planes. The surface stripes 188 are opposite the light-exiting plane 186. Since the cross-sections of the light-guided bars 182 is an irregular shape, light beam having an incident angle smaller than the critical angle is easy to generate. As a result, there is a loss of radiant flux and the rate of light energy utilization is not high. Further, since the device depends merely on the linear-varied oblique surface 184 to adjust the output radiant flux distribution, the uniformity is not sufficient.
In FIGS. 9 and 10, a linear light source 190 is composed of a light-guide bar 192 and alight source assembly 300. The light-guide bar 192 is constituted by the intersection of a prismatic column and an oblique plane. The cross-section of the prismatic column is an irregular shape (see FIG. 10). The oblique plane is an oblique curved surface, and the light-exiting planes 196 is an irregular prismatic column plane. The surface stripes 198 are coated with a reflective layer are on opposite the light-exiting plane 196. Additionally, a groove 199 formed by two oblique surfaces is provided. Since the cross-sections of the light-guide bars 192 is an irregular shape, light having an incident angle smaller than the critical angle is easy to generate. As a result, there is a loss of radiant flux, and the rate of light energy utilization is poor. Further, since the reflective layer is a simple belt-shape and the device depends merely on the oblique curved surface 194 and groove 199 to adjust the output radiant flux distribution, the uniformity is not sufficient.